Materials made of lignocellulosics such as paper are subject to a number of chemical, physical and biological hazards. Many chemical pulping, mechanical pulping, bleaching and sizing processes used in the last 200 years produce paper that has an inherent acidity that limits storage stability. Paper based materials are slowly deteriorating due to oxidative acid hydrolysis caused by both the acidity induced during manufacturing and further exposure to acidic air pollutants such as the oxides of sulfur and nitrogen and direct contact with acid containing materials, e.g. acidic paper and books.
The deterioration of paper and other lignocellulosic materials due to oxidative acid hydrolysis is widespread and losses to library and archival collections are estimated to be as high as one percent per year. Many manuscripts and books have become so embrittled that they are too fragile for normal use and must be removed from general circulation. The content of important books and papers is being microfilmed or digitized, but this only creates recordings of information. Stabilization of these works against acidity is necessary to preserve the original materials.
Deacidification or neutralization of the acids in paper is a known method of reducing the effects of oxidative acid hydrolysis. It is also well known to people skilled in the art that the deposition of an alkaline reserve into paper is also necessary to continue the protection of paper from the effects of acid degradation, U.S. Pat. Nos. 2,033,452 and 2,864,723. Earlier methods used soluble alkaline salts in polar solvents such as water. These methods could not be used on most materials due to damage caused by polar liquids. To circumvent these problems, most newer processes use either an alkaline gas or a soluble alkaline compounds in non-polar solvents to first neutralize acids present in paper then add an excess of the agent to provide an alkaline reserve into a material. One deacidification process treats paper by depositing insoluble small alkaline particles from a dispersion in an inert liquid. This provides not only an alkaline reserve, but also effectively stabilizes paper by providing alkalinity to neutralize acids in paper as they naturally diffuse through the paper.
A report titled "Mass Deacidification--An Update of Possibilities and Limitations" by H. J. Porck published in 1996, reviewed the leading methods of deacidification. The processes discussed in the Porck report include: the Diethyl Zinc, the Wei'To, the Bookkeeper, the Battelle, the FMC methods and an aerosol procedure in early development. The Diethyl Zinc method is gas based while all other deacidification methods reviewed in the Porck Report use alkaline compounds dissolved or suspended in liquids.
The Diethyl Zinc (DEZ) method disclosed in U.S. Pat. Nos. 3,969,549 and 4,051,276 utilizes a volatile metal alkyl, diethyl zinc. The DEZ process as described by Porck requires that paper be very dry so the material must be vacuum dried for 2-3 days before treatment. After drying, materials are exposed to gaseous diethyl zinc for 16-18 hours. Liquid diethyl zinc is pyrophoric and will ignite when exposed to air and reacts explosively with water. Consequently, the DEZ process was abandoned for reasons of safety and complexity in 1994.
The Wei'To method is one of the first organic liquid-phase mass deacidification processes and utilizes methoxymagnesium methylcarbonate (MMC) dissolved in an alcohol (methanol) which is then dispersed in fluorocarbons (CFCs) or hydrofluorocarbons (HCFCs). The process is described in U.S. Pat. Nos. 4,318,963, 3,676,055, 3,676,182 and 3,939,091. After the MMC neutralizes the acidity, it reacts with residual water in the paper to form magnesium oxide. The magnesium oxide then slowly reacts with ambient air and carbon dioxide to form magnesium hydroxide and basic magnesium carbonate which remain in the paper to form an alkaline reserve.
There are several drawbacks to the Wei'To method. The alcohol solvent used to dissolve the MMC may cause certain types of sensitive inks and dyes in the paper to run or feather. Alcohols are also very difficult to remove completely from paper and may leave a residual odor in the treated materials. Another drawback is that the CFCs and HCFCs have been restricted from manufacture by government regulations due to their ozone depletion potential. The Wei'To method also requires that books be dried for up to 36 hours prior to treatment and then reconditioned for two or three days after treatment.
The Bookkeeper method is the only deacidification method that utilizes alkaline particles of a basic metal oxide suspended in inert liquids with a suitable surfactant. The process is disclosed in U.S. Pat. No. 4,522,843 using CFC's and later in U.S. Pat. No. 5,409,736 when CFC's were replaced with perfluorocarbons. The '843 patent also teaches the treatment of paper or lignocellulosic material by direct impingement with an aerosol containing submicron alkaline particles and the use of electrostatic forces or a vacuum to draw the alkaline particles towards the paper. The Bookkeeper process deposits particulate magnesium oxide in paper which slowly reacts with water and carbon dioxide to form magnesium hydroxide and basic magnesium carbonate which resist the acidification process.
Drawbacks of the Bookkeeper method include complex treatment processes. The materials are subject to vacuum prior to treatment. The perfluorocarbon liquids used are expensive and the recovery of the perfluorocarbons requires both heat and vacuum for up to 18 hours. The long vacuum/heat liquid recovery process further dries the treated paper and the residual surfactants may reduce the abrasion resistance of the paper.
The German National Library, Frankfurt/Leipzig (Die Deutsche Bibliothek) developed deacidification methods described in U. S. Pat. Nos. 5,277,842 and 5,322,558, which are known as the "Battelle" method. The Battelle method utilizes magnesium titanium ethoxide (MTE) dissolved in hexadimethyl disiloxane. The MTE neutralizes acids then reacts with residual water in the paper to first form magnesium oxide which slowly reacts with additional water and carbon dioxide to form magnesium hydroxide and basic magnesium carbonate which resists the acidification process.
There are several drawbacks to the Batelle deacidification process. Books treated with the Battelle process must be dried for about 2 days prior to treatment and reconditioned for 3 weeks to reduce the residual treatment odor. The Batelle process uses hexadimethyl disiloxane, an expensive and highly flammable material that requires the use of explosion proof storage and treatment facilities. Further, to speed drying and solvent recovery, the Battelle process uses microwave energy drying that may damage any paper or books containing metallic inks, staples or stitching.
The FMC deacidification process is disclosed in U. S. Pat. Nos. 5,104,997, 5,208,072 and 5,264,243. The FMC process utilizes magnesium butyl glycolate (MGB) dissolved in heptane and claims that MGB both strengthens and deacidifies the treated materials. As with the other liquid based methods, the MGB reacts with traces of water to form the alkaline compounds magnesium oxide, magnesium hydroxide and basic magnesium carbonate to form an alkaline reserve.
The FMC process employs dielectric or radio frequency drying of materials prior to treatment as disclosed in U.S. Pat. No. 5,282,320. A post processing radio frequency heating step is also used to recover the heptane solvent. As with microwave energy, radio frequency heating may damage books or paper containing any metallic material.
Other drawbacks of the FMC process include the reaction of MGB and heptane solvents with the common book materials which adversely affects the materials. Residual glycolate in the FMC treated paper may form humectant glycols which cause the paper to swell and attract water vapor which change the texture of the paper. Heptane is also a flammable solvent requiring special storage and handling procedures.
The aerosol treatment process discussed in the Porck report deacidifies books by impinging alkaline particles in a stable aerosol cloud of submicron particles onto the pages. In order to prevent agglomeration the concentration of alkaline particles must be less than about a few milligrams/cubic foot to maintain stability. It is calculated that approximately five to ten cubic feet of this low concentration alkaline aerosol with must impact upon or past through a sheet of paper to deposit the required quantity of alkaline reserve(3% CaCO.sub.3). In addition, the deposition rate is very slow and the complex geometry of books makes it difficult to uniformly treat all pages of a book using aerosol impingement.
There are other deacidification processes disclosed in the literature. Almost all of these use gaseous deacidification agents. U.S. Pat. Nos. 3,472,611 and 4,927,497 disclose the use of volatile cyclohexylamine carbonate gas as a deacidification agent. U.S. Pat. Nos. 3,771,958 and 3,837,804 disclose the use of morpholine gas as a deacidification agent. U.S. Pat. Nos. 3,771,958 and 5,393,562 both disclose the use of ammonia gas as a deacidification agent. A problem with deacidification agents as a gas or vapor is that they do not remain in paper to provide the alkaline reserve required for long term protection from future acid attack. Further, many volatile amines leave residual odors in treated books and may pose health risks. Many gas based deacidification processes have been abandoned due to safety problems, high costs, residual odor and reports of damage to materials treated.
An alternate deacidification method to either gas or liquid treatments is disclosed in U.S. Pat. No. 5,433,827. The '827 patent process interlaces paper with sheets of base impregnated paper which allows mobile alkaline metal cations to neutralize acids in the paper. The books are then subjected to heat and pressure at a relatively high humidity over a number of days during which time alkaline metal cations migrate through the paper. This method is very labor intensive, can damage bindings and does not deposit an adequate alkaline reserve to prevent damage from future exposure to acids.
What is needed is a simple deacidification process that can treat books in a reasonable amount of time and prevent damage from future acid exposure without the use of hazardous materials.